Microcavity plasmas, plasmas confined to a cavity with a characteristic spatial dimension <1 mm, have several distinct advantages over conventional, macroscopic discharges. For example, the small physical dimensions of microcavity plasma devices allow them to operate at gas or vapor pressures much higher than those accessible to a macroscopic discharge such as that produced in a fluorescent lamp. When the diameter of the microcavity is, for example, on the order of 200-300 μm or less, the device is capable of operating at pressures as high as atmospheric pressure and beyond. In contrast, standard fluorescent lamps operate at pressures typically less than 1% of atmospheric pressure. Also, microplasma devices may be operated with different discharge media (gases, vapors or combinations thereof) to yield emitted light in the visible, ultraviolet, and infrared portions of the spectrum. Another unique feature of microplasma devices, the large power deposition into the plasma (typically tens of kW/cm3 or more), is partially responsible for the efficient production of atoms and molecules that are well-known optical emitters. Consequently, because of the properties of microplasma devices, including the high pressure operation mentioned above and their electron and gas temperatures, microplasmas are efficient sources of optical radiation.
Microcavity plasma devices have been developed over the past decade for a wide variety of applications. An exemplary application for an array of microplasmas is in the area of displays. Since single cylindrical microplasma devices, for example, with a characteristic dimension (d) as small as 10 μm have been demonstrated, devices or groups of devices offer a spatial resolution that is desirable for a pixel in a display. In addition, the efficiency for generating, with a microcavity plasma device, the ultraviolet light at the heart of the plasma display panel (PDP) significantly exceeds that of the discharge structure currently used in plasma televisions.
Early microplasma devices were driven by direct current (DC) voltages and exhibited short lifetimes for several reasons, including sputtering damage to the metal electrodes. Improvements in device design and fabrication have extended lifetimes significantly, but minimizing the cost of materials and the manufacture of large arrays continue to be key considerations. Also, more recently-developed microplasma devices excited by a time-varying voltage are preferable when lifetime is of primary concern.
Microcavity plasma devices have been made in a variety of materials, including molybdenum, ceramics, silicon, and polymer/metal film structures. Microcavities have been made by a variety of techniques, including etching, mechanical drilling and laser ablation. Each of these fabrication techniques has one or more drawbacks. For example, with laser ablation and mechanical drilling, the size of the microcavity is typically limited to about 50 μm, with smaller sizes more difficult to make. Additionally, the cross-sections of microcavities formed by ablation and drilling methods are not completely uniform. In the case of drilling, mechanical wear of drill bits and mechanical tolerances prevent the achievement of accurate dimensional control and repeatability. Also, the serial nature of the cavity drilling procedure makes the time and cost of processing prohibitive for the production of large arrays of microcavity plasma devices. Another drawback is that the techniques used for microcavity formation are not readily adaptable to produce other features, e.g., channels.
Research by the present inventors and colleagues at the University of Illinois has pioneered and advanced the state of microcavity plasma devices. This work has resulted in practical devices with one or more important features and structures. Most of these devices are able to operate continuously with power loadings of tens of kW-cm−3 to beyond 100 kW-cm−3. One such device that has been realized is a multi-segment linear array of microplasmas designed for pumping optical amplifiers and lasers. Also, the ability to interface a gas (or vapor) phase plasma with the electron-hole plasma in a semiconductor has been demonstrated. Fabrication processes developed largely by the semiconductor and microelectromechanical systems (MEMs) communities have been adopted for fabricating many of these microcavity plasma devices.
Use of silicon integrated circuit fabrication methods has further reduced the size and cost of microcavity plasma devices and arrays. Because of the batch nature of micromachining, not only are the performance characteristics of the devices improved, but the cost of fabricating large arrays is also reduced. The ability to fabricate large arrays with precise tolerances and high density makes these devices attractive for display applications. While representing an important step in the development of microdischarge devices, micromachined fabrication approaches also have limitations. One limitation is that the size of an individual array is limited to the size of the silicon substrate. Second, the cost of a device is determined not only by the substrate cost, but also by the cost of performing an expensive series of photolithographic, thin film deposition, and etching steps on each wafer in the batch. Finally, although silicon wafers are a convenient substrate due to the wide range of processing options that are available, silicon is an optically opaque material, and is therefore not suitable for applications such as heads-up displays or applications requiring lateral propagation or coupling of visible light between microcavity plasma devices in an array.
This research by present inventors and colleagues at the University of Illinois has resulted in exemplary practical devices. For example, semiconductor fabrication processes have been adopted to demonstrate densely packed arrays of microplasma devices exhibiting uniform emission characteristics. Arrays fabricated in silicon comprise as many as 250,000 microplasma devices in an active area of 25 cm2, each device in the array having an emitting aperture of typically 50 μm×50 μm. It has been demonstrated that such arrays can be used to excite phosphors in a manner analogous to plasma display panels, but with values of the luminous efficacy that are not presently achievable with conventional plasma display panels. Another important device is a microcavity plasma photodetector that exhibits high sensitivity. Phase locking of microplasmas dispersed in an array has also been demonstrated.
The following U.S. patents and patent applications describe microcavity plasma devices resulting from these research efforts. Published Applications: 20050148270-Microdischarge devices and arrays; 20040160162-Microdischarge devices and arrays; 20040100194-Microdischarge photodetectors; 20030132693-Microdischarge devices and arrays having tapered microcavities; U.S. Pat. No. 6,867,548-Microdischarge devices and arrays; U.S. Pat. No. 6,828,730-Microdischarge photodetectors; U.S. Pat. No. 6,815,891-Method and apparatus for exciting a microdischarge; U.S. Pat. No. 6,695,664-Microdischarge devices and arrays; U.S. Pat. No. 6,563,257-Multilayer ceramic microdischarge device; U.S. Pat. No. 6,541,915-High pressure arc lamp assisted start up device and method; U.S. Pat. No. 6,194,833-Microdischarge lamp and array; U.S. Pat. No. 6,139,384-Microdischarge lamp formation process; and U.S. Pat. No. 6,016,027-Microdischarge lamp.
U.S. Pat. No. 6,541,915 discloses arrays of microcavity plasma devices in which the individual devices are mounted in an assembly that is machined from materials including ceramics. Metallic electrodes are exposed to the plasma medium which is generated within a microcavity and between the electrodes. U.S. Pat. No. 6,194,833 also discloses arrays of microcavity plasma devices, including arrays for which the substrate is ceramic and a silicon or metal film is formed on it. Electrodes disposed at the top and bottom of microcavities contact the discharge medium. U.S. Published Patent Application 20030230983 discloses microcavity plasmas produced in low temperature ceramic structures. The stacked ceramic layers are arranged and micromachined so as to form cavities and intervening conductor layers excite the plasma medium. U.S. Published Patent Application 20020036461 discloses hollow cathode discharge devices in which electrodes contact the plasma/discharge medium.
Microcavity plasma devices have also been fabricated in glass that can be etched by photolithography techniques. See, e.g., Kim, S.-O., and J. G. Eden, IEEE Photon. Technol. Lett. 17, 1543 (2005). As with silicon fabrications, the array size is limited to the size of the substrate and the surface area that can be contiguously patterned by photolithography. Array cost is dominated by the cost of performing multiple photolithography steps.
The development of microcavity plasma devices continues, with an emphasis on the display market and the biomedical applications market. The ultimate utility of microcavity plasma devices in displays will hinge on several critical factors, including efficacy (discussed earlier), lifetime and addressability. Addressability, in particular, is vital in most display applications. For example, for a group of microcavity discharges to comprise a pixel, each microplasma device must be individually addressable.
Current flat panel display solutions suffer from a number of drawbacks. Flat panel display technologies that have been widely adopted include liquid crystal displays (LCDs) and plasma display panels (PDPs). These technologies have been widely adopted for large screen formats such as televisions. LCDs are also used in computer displays. Compact electronic devices such as personal digital assistants (PDA) also benefit from high contrast, bright, high resolution displays.
Plasma display panels are in widespread use as high definition displays. While the basic technology for PDPs dates back to the 1960s, the materials, design, and manufacturing methods developed for plasma displays have evolved over the past two decades to enable the high resolution, long lifetime, and high brightness microplasma arrays for PDPs available today. Individual PDP cells (three cells to a pixel: red, blue, green) tend to have characteristic dimensions (d)>300 μm, and pd (pressure x electrode separation) scaling design rules result in total gas pressures of nominally 400-500 Torr. Consequently, PDPs must be sealed hermetically within an enclosure (normally, glass) that is sufficiently robust (i.e., thick) to withstand atmospheric pressure and this factor is primarily responsible for the undesirably large weight of these displays.
Practical designs that would permit the use of microcavity plasma devices would likely alter the landscape of the flat panel display industry. Compared to standard flat panel display technologies, microplasma devices offer the potential of smaller pixel sizes, for example. Small pixel sizes correlate directly with higher spatial resolution. In addition, tests have shown that microplasma devices convert electrical energy to visible light at a higher efficiency than that available with conventional pixel structures in plasma display panels.